Free cutting alloys



Patented Apr. 13, 1937 AUNITED- STATES PATENT "OFFICE Ohio, assignors toAmerica, Pittsburgh,

Pennsylvania Aluminum Company of Pa., a corporation'of No Drawing.Application December 28, 1935, Serial No. 56,543

5. Claims.

The invention relates to aluminum base alloys and is particularlyconcerned with aluminum base alloys containing copper, magnesium andsilicon.

, Aluminum base alloys containing between 3 5 per cent and 12 per centof copper have beenwldely used heretofore. The copper imparts goodcasting characteristics and increases the tensile strength, yieldstrength, and hardness of the alloy. In the lower portion of the copperrange the alloy may be mechanically deformed by the well knowncommercial processes such as rolling, forging, drawing, or extrusion.Alloys containing more than about 6 per cent copper are gen erally usedin the cast' condition. Throughout 15 the entire range of 3 to 12 percent copper, however, the alloys are susceptible to variations andimprovements in their physical properties by thermal treatments.

Alloys of the foregoing type are greatly improved in castingcharacteristics and strength through the addition of from 0.5 to 2.5 percent of silicon and 0.1 to 1 per cent of magnesium. The silicon serveschiefly to improve the casting and physical qualities of the alloy whilethe magnesium tends to make it more responsive to solution heattreatment and induces a more rapid aging after such heat treatment. Ourinvention, which is described hereinbelow, is directed to improving themachining quality of such aluminum-copper-magnesium-silicon alloys.

Since all commercial aluminum base alloys contain some silic'on as animpurity, usually less than, or not greatly in excess of, 0.5 per cent,it is to be understood that the amount of silicon discussed herein, andhereinafter claimed, represents the total quantity of silicon present,and that the silicon content of the aluminum used should be known inorder to provide a basis for determining the eventual composition of thealuminumcopper-magnesium-silicon alloy.

There are, however, some applications whereinaluminum-copper-magnesium-silicon alloys as hereinabove disclosed mightbe conveniently and profitably used except for an inherent disadvantagewhich militates against their usein the production of certain articlesrequiring exact-.

ing machining operations. Mechanical cutting operations such asdrilling, shaping, or lathe-cutting are successfully carried out only byusing cer- 0 tain precautions which increase the cost of production andperhaps favor the choice of another metal or alloy which can be machinedmore readily but which is not so desirable in other respects, as forexample, in physical properties. When alloys are difficult to machinethis disadvantage becomes evident, in many cases, through rapid wear ofthe cutting tool edge, so that frequent tool re-sharpening is required.Despite continual lubrication the machined surface is rough andirregular, and the chip has a tendency to form a continuous curl orspiral which often fouls the tool or the moving parts of the machine. Itis immediately apparent that there is need for an alloy of good workingcharacteristics and satisfactory physical properties, yet possessingsuch favorable machining properties that the complete machiningoperations may be performed economically and successfully, and may beproductive of a pleasing surface appearance.

Accordingly an object of our invention is the provision of an aluminumbase alloy containing from about 3 per cent to about 12 per cent ofcopper, from about 0.1 to about 1 per cent of magnesium, and from about0.5 per cent to about 2.5 per cent of silicon which may be readily andeconomically machined.

Our invention resides in the discovery that the foregoing object iseffected by the addition of two or more of the elements lead, tin,thallium, cadmium or bismuth. The aluminum base alloys to which theseelements are added in the proportions specified below, are known as freecutting or free machining alloys because they can be machined morerapidly than similar alloys without these elements and yet have as goodorbetter finished surface. After an extended series of investigations wehave discovered that these five metals when added to aluminum basealloys, form a class of alloying elements by reason of their 3 favorableeffect upon the machining properties of these alloys. In recognition ofthis effect we term lead, tin, thallium, cadmium and bismuth freemachining elements. We have further discovered that the simultaneouspresence of two or more of these elements is productive of animprovement in free machining characteristics which is considerablygreater than that caused by the presence of the same total amount of asingle free machining element. For example, the additionof 0.5 per centof lead and 0.5 per cent of bismuth to an aluminum base alloy containingabout 5 per cent of copper, 0.5 per cent of magnesium, and 1.25 per centof silicon effects a greater improvement in machining quality than doesthe addition of 1.0 per cent of either lead or bismuth singly.

These five elements, we believe, are unique with respect to their effecton the machining characteristics of aluminum-copper-magnesium-siliconalloys. It is a fortunate circumstance, therefore, that they are also ofrelatively low melting point, a fact which makes possible their additionto molten aluminum in the pure state, without the intervention ofso-called "rich alloys". As a matter of fact, we have observed that ofall the metals whose melting point is lower than about 327 C., themelting point of lead, the five elements we have selected are the onlyones which are commercially suitable and that impart free cuttingcharacteristics but do not have undesirable effeet on the fundamentalphysical properties of the base alloy.

The total amount of free machining elements should not be less thanabout 0.05 per cent since below this amount there is scarcely anyadvantageous effect. We have determined that a maximum limit of about 6per cent total of two or 2 more of the free machining elements issuiiicient for satisfactory commercial results, since although the freemachining effect persists beyond this amount, certain of the otherphysical properties may be unfavorably affected.

Aluminum-copper-magnesium-silicon allo ys containing two or more of thefree machining elements lead, tin, thallium, cadmium and bismuth may bemachined more rapidly, with less tool wear, less tool sharpening, betterquality of chip and better machined surface than the same base alloyswithout the free machining additions, and in fact better than the samebase alloys containing an equivalent total amount of a single freemachining element.

Since aluminum base alloys containing from 3 to 12 per cent of copperhave a wide variety of applications we list several alloys each of whichmay be said to be preferred for a particular purpose. As an alloy formechanical deformation 4 we suggest an aluminum base alloy containing 5per cent of copper, 0.5 per cent of magnesium, 1.25 per cent of silicon,and a total of 1 per cent of free cutting constituents, for example, 0.5per cent of bismuth and 0.5 per cent of cadmium, the

balance being aluminum. For an alloy with excellent castingcharacteristics to be used in the unworked condition we suggest an alloycontaining 10 per cent of copper, 0.5 per cent of magnesium, 2 per centof silicon and a total of 3 per cent of free machining elements, thebalance being aluminum.

For certain purposes, notably the improvement of tensile strength,hardness and grain structure, the alloys as hereinabove disclosed may beimproved by the addition of one or more of the group of elementscomposed of molybdenum, vanadium, titanium, tungsten, zirconium, andchromium. From 0.05 to 1 per cent of any one of these elements may beused alone, but if more 00 than one is employed the total amount shouldnot exceed about 2 per cent.

It is characteristic of the five elements lead, tin, thallium, cadmium,and bismuth that they form with aluminum a series of alloys of limited05 liquid solubility. We have reason to believe that the free machiningelements are the only elements which exhibit this characteristic, withthe possible exception of several metals which are not regarded ashaving any commercial promise as 70 additions to aluminum base alloys.Within the range disclosed and claimed however the free machiningelements may be added without unusual diillculty. We suspect that thischaracteristic feature of the disclosed elements may be one of thesignificant factors which contribute to their free machining eflect.We'believe that this effect is further strengthened by distributing thefree machining constituent relatively homogeneously throughout the solidmatrix, since these free machining constituents are also practicallyinsoluble in the solid aluminum base.

The free machining alloys which have been described hereinabove may besubjected to the thermal treatments well known in the art to improvetheir strength and hardness. We have found that a solution heattreatment and subsequent aging do not impair the free machining qualityof the alloys and in many instances the treatment even tends to improvethis property. For many purposes a relatively high strength and hardnessare necessary to the successful performance of the machined article andhence the alloy must be heat treated. This treatment is generallyapplied prior to the machining operation.

As hereinabove indicated the free machining elements, by reason of theirlow melting point, may be added to the molten aluminum alloy in puremetallic form. However, since some dimculty may be encountered inintroducing them in the higher percentages of our disclosed range weprefer to use the method which is more fully described in U. S. PatentNo. 1,959,029, issued March 15, 1934; Briefly it involves heating themelt to a somewhat higher temperature than is customary, and vigorouslystirring it in excess of a critical period of time.

The term aluminum as used herein and in the appended claims embraces theusual impurities found in aluminum ingot of commercial grade or pickedup in the course of the ordinary handling operations incident to meltingpractice.

We claim:

1. An aluminum base alloy consisting of about 5 per cent copper, 0.5 percent magnesium, 1.25 per cent silicon, 0.5 per cent lead and 0.5 percent bismuth, the balance being aluminum.

2. An aluminum base alloy consisting of about 5 per cent copper, 0.5 percent magnesium, 1.25 per cent silicon, 0.5 per cent bismuth and 0.5 percent cadmium, the balance being aluminum.

3. A free cutting alloy containing from 3 to 12 per cent of copper, from0.1 to 1 per cent of magnesium, from 0.5 to 2.5 per cent of silicon anda total of from 0.05 to 6 per cent of at least two of the elements fromthe following metals, lead, tin, thallium, cadmium, and bismuth, toimprove its machining properties, the balance being substantiallyaluminum.

4. A free cutting alloy containing from 3 to 12 per cent of copper, from0.1 to 1 per cent of magnesium, from 0.5 to 2.5 per cent of silicon,from 0.05 to 2 per cent of hardening metal from the group composed ofmolybdenum, vanadium, titanium, tungsten, zirconium and chromium, and atotal of from 0.05 to 6 per cent of at least two of the elements fromthe following metals lead, tin, thallium, cadmium, and bismuth, toimprove its machining properties, the balance being substantiallyaluminum.

5. An aluminum base alloy consisting of about 5 per cent copper, 0.5 percent magnesium, 1.25 per cent silicon, 0.5 per cent cadmium, and 0.5 percent lead, the balance being aluminum.

-LOUIS W. KEMPF. WALTER A. DEAN.

